1. Field of the Invention
This invention relates to a self-curable coating composition for cathode-precipitating electrodeposition.
2. Prior Art
Generally, a resin having a certain basic group turns into a cationic resin in contact with water and when applied to electrodeposition, precipitates on a cathode. Cathode-precipitative coatings of this type can eliminate the tendency of the metal to be coated to dissolve in a coat bath and related problems which are encountered with conventional cathode-precipitative coatings prepared from resins having acid groups and rendered water-soluble by neutralization with bases.
The present inventors have previously found that the introduction of tertiary amino groups to high molecular-weight compounds having unsaturated groups or low polymerization degree polymers having carbon-carbon double bonds such as liquid polybutadienes will result in a resin having excellent film properties eligible for use in cathode-precipitating electrodeposition. See Japanese Laid-Open Patent Publication Nos. 51-119727, 52-147638 and 53-16048. Coating compositions having this type of resin are curable primarily by oxidative polymerization of the unsaturated groups in the resin but would require a relatively high baking temperature to effect curing for a practical length of time. Dryers such as salts of water-soluble metals like manganese may be used to cure the coated film at a lower backing temperature, in which instance however considerable amounts of such dryers are required. This would in turn result in defective coated surface finish, reduced corrosion-resistance and other objectionable phenomena.
The present inventors have also previously found that satisfactory coating compositions for cathode-precipitating electrodeposition can be obtained by adding water-dispersible modified epoxy resins having secondary amines to water-dispersible resins having carbon-carbon double bonds and tertiary amino groups, the resulting compositions being highly resistant to corrosion and stable in liquid form. See Japanese Laid-Open Patent Publication Nos. 55-60572 and 55-60851. However, these coating compositions were found to be poor in solvent-resistance due to insufficient oxidative polymerization of the modified epoxy resins used.
The present inventors have further previously found that excellent results as regards corrosion-resistance, solvent-resistance and coated film properties can be obtained by adding water-undispersible modified epoxy resins having certain .alpha.,.beta.-unsaturated carboxylic acids to water-dispersible resins having carbon-carbon double bonds and tertiary amino groups. See Japanese Laid-Open Patent Publication Nos. 54-69931, 55-55473 and 54-117857. The coating compositions disclosed therein are curable primarily by oxidative polymerization of the unsaturated groups in the resin and are found to be highly resistant to corrosion, but are not satisfactorily stable in liquid coating form.